(1) Field of the Invention
This invention relates to a method of determining an exposure condition which is corrected in accordance with variation of a light source.
(2) Description of the Invention
Generally, a printer of a photographic printing apparatus comprises a light source including a halogen lamp and the like an optical system including a CC filter and the like. Light emitted from the light source is transmitted through the optical system and irradiated onto a photographic paper after passing through a negative film. Thus, an image recorded on the negative film is printed on to the photographic paper. The optical system is controlled in accordance with a predetermined exposure condition. For example, the exposure condition is determined as follows. A so-called eye pattern negative is prepared as a reference film. A plurality of images for use in determining the exposure condition are recorded on the reference film. Each of the images comprises a central region corresponding to a gray-colored object and a surrounding region corresponding to an average object color. A density value of the reference film is preliminarily measured as a light amount of transmission light passing through the reference film and memorized as a reference density value. Next, a density value of a negative film to be printed is measured. A difference is calculated between the reference density value and the density value of the negative film to be printed. The exposure condition is determined in accordance with the difference thus calculated.
The printer is capable of performing a printing operation on a plurality of types of negative films. A plurality of types of reference films are prepared in correspondence to the types of negative films to be printed. In this connection, the printer is provided with a plurality of memory areas or channels in one-to-one correspondence to the types of the reference films. The density values of the reference films preliminarily measured are individually stored in the corresponding channels as the reference density values. The exposure condition is determined with reference to a corresponding one of the reference density values that is stored in a particular channel corresponding to the type of the negative film to be printed.
In the above-described printer, the light amount emitted from the light source often varys due to a change of the lamp in the light source, aging of the lamp, a change of a position of the lamp (change of a position of a filament), a change of a reflector behind the lamp, adjustment of the CC filter, a variation of a source voltage, and the like. The variation of the light amount emitted from the light source will hereinafter be referred to as light source variation. Upon occurrence of the light source variation, a relationship between a position of the filter and an amount of transmission light passing through the filter is changed. This results in unfavorable variation in measurement of the density values. It is therefore impossible to determine an accurate exposure condition when the light source variation occurs between the time of measurement of the reference density value and the time of measurement of the density value of the negative film to be printed.
In view of the above, the present inventor has already disclosed a light source variation correction method capable of obtaining an exposure condition from which the influence of light source variation is removed (see Japanese Patent Prepublication No. 307737/1989). in this light source variation correction method, a current density value Dx of a specific reference film is measured after the occurrence of the light source variation. An initial density value Di (i=x) is preliminarily measured and stored in a channel corresponding to the specific reference film. A difference .delta.i=Di (i=x)-Dx is calculated between the initial density value Di (i=x) and the current density value Dx. All initial density values Di (i=1 through n) preliminarily stored in the respective channels are renewed with reference to the difference or a correction value .delta.i. With the reference film set in the printer, a calibration curve is prepared which represents a relationship between the position of the CC filter and the amount of transmission light. Thus, an exposure condition is obtained which is corrected in accordance with the light source variation.
In the meanwhile, it takes a long time to preliminarily measure the initial density values Di(i=1 through n) storing in the respective channels for a plurality of types of reference films. Accordingly, measurement of the initial density values for the plurality of types of reference films may often be carried out for several days. Before measurement is completed for all of the reference films, the light source variation is possibly caused by the aging of the light source. The changing of the light source, or the like. In this event, the initial density values Di measured under different conditions (namely, different light amounts of the light source) are stored in the respective channels. If the initial density values Di are uniformly corrected according to the above-mentioned conventional method by the use of the correction value .delta.i which is obtained under a particular light amount of the light source, the initial density values Di can not be properly corrected because they are measured under different conditions.
In the above-mentioned conventional method, measurement of the density values must be repeatedly carried out for the specific reference film each time light source variation occurs. Such repeated measurement of the reference film results in the fading of the reference film. Consequently, it is impossible to accurately compensate for the light source variation.